Photonic integrated circuits (PICS) based on waveguide technology to manipulate light signals on a chip level can be employed in information processing, communication and optical sensing etc. For photonic integrated circuits, there are two fundamental but very important requirements 1) small waveguide cross-section with a strong light confinement so that one single chip can contain multiple integrated optical devices, and 2) low loss optical coupling to/from the photonic chip for the purpose of chip interconnection.
Nanophotonic waveguide, based on for example silicon or indium phosphide (InP) semiconductor, has a high refractive index contrast between the core and cladding, which leads to a strong light confinement with a sub-wavelength cross-section and a corresponding sub-wavelength mode field diameter. This makes it superior in dense photonic integration and promising for photonic system/network on chip. On the other hand, optical fiber is widely used for light guiding and interconnection, e.g., in optical communication. The typical mode field diameter of an optical fiber is a few micrometers (e.g., it is ˜10.5 μm for a standard single mode fiber at a wavelength of 1.55 μm). Therefore, this mismatch of mode field profiles between the nanophotonic waveguide and optical fiber leads to a high coupling loss when interconnection of photonic chips is required, for example to optical fibers.
As an example, FIG. 1A shows schematic cross-sectional views of a silicon nanophotonic waveguide 100 and a single mode optical fiber 120 of the prior art, to illustrate the high coupling loss between the nanophotonic waveguide 100 and the single mode fiber 120. As shown in FIG. 1A, the silicon nanophotonic waveguide 100 includes a waveguide core 102 and a silicon-on-insulator substrate 104 including a buried silicon oxide 106 on top of a silicon substrate 108. The buried silicon oxide 106 acts as a cladding. For illustration purposes, the waveguide core 102 of the silicon nanophotonic waveguide 100 may have a width, w, of about 400 nm and a height, h, of about 300 nm. The refractive index for the silicon waveguide core 102 is about 3.45 and the refractive index for the buried silicon oxide cladding 106 is about 1.45, at the wavelength of 1.55 μm.
As a comparison, FIG. 1A also shows the cross-sectional view of a standard single mode fiber 120 including a fiber core 122 surrounded by a cladding 124. For illustration purposes, the radius, r, of the fiber core 122 is about 4.15 μm.
FIG. 1B shows a plot 130 of the mode field profile 132 of the silicon nanophotonic waveguide 100 (FIG. 1A) and the mode field profile 134 of the single mode optical fiber 120 (FIG. 1A), along the y-direction as indicated in FIG. 1A at the wavelength of 1.55 μm. As shown in FIG. 1B, the two mode field diameters or mode field profiles 132, 134, are distinctly different. Therefore, a direct butt coupling between these two optical modes may cause a coupling loss of about 20 dB due to this mismatch, even when perfect alignment between the nanophotonic waveguide 100 (FIG. 1A) and the optical fiber 120 (FIG. 1A) is achieved.
In order to reduce the coupling loss between a nanophotonic waveguide and an optical fiber, a simple and commonly used setup in laboratory involves a discrete lens with a typical millimeter size between the optical fiber and the waveguide to focus the light from the optical fiber to the nanophotonic waveguide (or expand the light beam from the nanophotonic waveguide to match the fiber). For example, FIG. 2 shows a schematic side view of a coupling arrangement 200 of the prior art, using an objective lens 202 positioned in between a waveguide 204a and a single mode optical fiber 206a. The nanophotonic waveguide 204a is placed on a waveguide holder 206a, the optical fiber 206a is placed within a fiber holder 206b and the objective lens 202 is placed in a corresponding lens holder 206c. The coupling arrangement 200 is configured such that the waveguide core 204b of the waveguide 204a and the fiber core 206b of the optical fiber 206a are aligned with each other and in addition, aligned to an optical axis, as represented by the dotted line 208, of the objective lens 202.
However, such a coupling method as illustrated by the coupling arrangement 200 of FIG. 2 has two main problems. One of the problems involves complicated assembling requirement and high packaging cost. The coupling arrangement 200 of FIG. 2 is a discrete alignment in free space, which involves 5-axis adjusters or 5-freedom adjusters for an optical fiber. FIG. 3 shows a schematic view of an optical fiber 300, illustrating a five-axis adjustment in order to adjust or align the vertical-axis y 302, the optical axis z 304, the horizontal axis x 306, the pitch angle 308 and the yaw angle 310. Therefore, proper alignment in these 5 axes is required, increasing the complexity of the optical coupling process.
In addition, as shown in FIG. 1B, a nanophotonic waveguide has a sub-wavelength spot size. However, an objective lens used for focusing light typically has a millimeter-scale size and focusing distance. Therefore, assembly based on this alignment in free space is complicated, which increases the packaging cost significantly and is not suitable for mass production.
Another problem is degraded performance of optical lens for sub-wavelength optical beams. Generally, two types of optical lens are available for applications in fiber optics, such as a conventional objective lens or a conventional graded refractive index or gradient-index lens (GRIN lens) for expanding or focusing light beams. However, both the conventional objective lens and the conventional GRIN lens have limited numerical apertures. As numerical aperture determines the resolution that can be achieved by a lens, both the conventional objective lens and the conventional GRIN lens do not have good resolution. Therefore, these available conventional lenses (eg. a GRIN lens based fiber collimator) can work reasonably well for expanding a light beam with a micro/millimeter scale beam size, but suffers from severe aberration when the light beam has a sub-wavelength spot size. As a result, coupling systems using these conventional lenses may not achieve much improvement in the coupling efficiency.
Optical coupling to a nanophotonic waveguide using, for example, an asymmetric GRIN lens with a low refractive index contrast, is no longer adequate due to the fact that the low refractive index contrast gives a low numerical aperture (NA) for the lens and therefore is not able to bend the light rays that are propagating at large angles around. In addition, even at high NA, a conventional GRIN lens with a parabolic refractive index profile is no longer adequate due to severe spatial aberration of the light beams at the focal plane, caused by the large-angle light rays of the focusing beam. Therefore, the use of the conventional low-index-contrast GRIN lens or the high-index-contrast GRIN lens with the conventional parabolic profile for optical coupling do not lead to a good coupling efficiency between an optical fiber and a nanophotonic waveguide due to the above-mentioned issues.
While photonic integrated circuits or photonic chips have been under fast development, benefiting from the fabrication techniques developed in the micro-electronics industry, the above-mentioned optical coupling problems present a barrier in engineering and commercialization of the photonic chips.
In order to achieve a high coupling efficiency between a nanophotonic waveguide and an optical fiber, various coupling schemes have been proposed and demonstrated numerically or experimentally in the prior art. These include using an inverse taper, a grating structure, and/or a bi-level mode converter. While these proposed structures may improve the coupling efficiency, specific requirements are required in order to achieve the necessary coupling efficiency. For example, the coupling scheme using an inverse tapered nanophotonic waveguide requires a thick buried oxide layer, while the approach using the grating structure requires a vertical or a tilted fiber-alignment.